This invention relates to horizontal deflection circuits.
In television receivers, the ultor high voltage accelerating potential for beam current in many instances is derived at least in part from horizontal retrace pulses appearing across a tertiary winding of a horizontal output transformer. For transistorized horizontal deflection circuits, the magnitude of the retrace pulses and thus the magnitude of the high voltage is a function of both the B+ operating voltage supplied to the horizontal deflection circuit and of the parameters of the resonant retrace circuit formed by the total retrace capacitance and inductance in resonance. A decrease in either B+ voltage or resonant frequency will tend to reduce the retrace pulse magnitude and thus reduce the high voltage.
The width of each raster line is proportional to the peak-to-peak trace current and is inversely proportional to the square root of the high voltage. A decrease in B+ voltage will proportionally decrease both peak-to-peak trace current and high voltage. However, since raster width is inversely proportional to the square root of the high voltage, a decrease in B+, without further compensation, will produce a decrease in raster width and possibly also an undersirable visible underscan condition.
To protect against exposure to harmful X-radiation, many television receivers include a high voltage protection circuit for sensing the high voltage and disabling normal operation of the horizontal deflection circuit if the high voltage increases above a predetermined value. The tripping voltage, or value of high voltage above which the protection circuit will disable the horizontal deflection circuit, is selected low enough so that the disabling will occur well before any harmful radiation is produced under all beam loading and other environmental conditions.
Manufacturing tolerances in the production of the electrical components comprising the protection circuit will produce some variation in the value of the tripping voltage from one assembled television receiver to another. In some assembled receivers, the deviation of the component values are such as to decrease the tripping voltage to a point that nuisance tripping will occur, that is, during operation of the television receiver, disabling of the horizontal deflection circuit will occur even though no possibility exists for generating harmful X-radiation.
It is desirable, therefore, to design a horizontal deflection circuit such that by means of simple corrective steps, nuisance tripping can be eliminated from those receivers subject to such a problem. A feature of the invention is to eliminate the nuisance tripping by decreasing the B+ operating voltage. To maintain a constant raster width, the circuit values in the horizontal deflection circuit should also be changed. The basic operation of the horizontal deflection circuit, however, should remain unchanged. The corrective steps to be taken should require much less effort than that of replacing all of the high voltage protection circuit components that produced the nuisance tripping condition.